A lengthy series of experiments has resulted in the production of a high-purity source of Rb which was used in the internal-source method of pair-production cross-section measurements. The 1.077-MeV y ray in the decay of Rb was used to measure the absolute pair-production cross section o. pp --1.92+0. 11 mb in a target of Z =82. This is the lowest-energy measurement for a target with Z &32 and shows a significant departure from the theoretical value of 1.34 mb. This is 6% lower than our result published earlier because of a far more detailed correction for pair production by inner bremsstrahlung radiation.
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